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There are a variety of ways that oak canopy can be measured. From aerial photos, you can use a dot grid to determine what percent of the dots fall on canopy, and what percent falls on ground. That would give percent canopy cover for a given stand. It’s also possible to run a series of transects. I have used 100 foot tapes and determined where oak canopy overlaps the transect. The number of feet covered by canopy would translate to percent canopy cover. A third way would be to measure the crown radius in two dimensions of a tree in fixed radius plots (1/10 or 1/5 acre plot). Using the area of a circle formula for the average radius (pi x r squared) gives square feet of canopy cover per tree. Summing up all the trees on the plot, expanding to a per acre basis, and dividing by 43,560 square feet would give percent canopy cover. There are some measurement devices, looking like a periscope that can be used to determine what percent of observations intercepts tree canopy.  I personally prefer to use the line transect approach to get percent canopy. There is an Extension publication that has some good ideas: "Monitoring California’s Annual Rangeland Vegetation", Catalog # 21486, $3.00 / EACH (http://Anrcatalog.ucdavis.edu/GrasslandsHardwoodRangelands/21486.aspx)